1. Technical Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus wherein an image density control method is adopted for varying developing conditions corresponding to an image density signal which is based on an image density detected so that the image density can be stabilized.
2. Description of Related Art
In an image forming apparatus which is arranged for forming a color image, for instance, color is reproduced by mixing toner of three primary colors of cyan, magenta and yellow. It is, therefore, necessary that image density (amount of toner adhesion) of the three primary colors to a photoconductor should be stabilized in order to perform satisfactory color reproduction.
In a conventional image forming apparatus, the following methods are known for stabilizing the amount of toner adhesion.
(1) Toner Density Control Method (ATDC)
In this method, toner density in a developer is detected by a variation in inductance of a coil, and corresponding to its detection signal, an amount of toner replenishment is controlled so that a toner mixing ratio to the developer can be set at a fixed value. In this method, however, only a toner mixing ratio is kept at a fixed value, and it cannot correspond with a variation in electric charge of a developer caused by a variation in surroundings. The amount of toner adhesion can not, therefore, be controlled at a fixed value to a variation in surroundings. Even under the same surroundings with the same amount of electric charge, an amount of toner adhesion is varied if other conditions (electric charge on the surface of photoconductor, sensitivity, amount of exposure and the like) are changed.
(2) Image Density Control Method (AIDC)
In this method, a standard pattern is formed on a photoconductor and its density is detected to control the amount of toner replenishment corresponding to the detection signal. Even if the amount of electric charge of a developer and other conditions are changed, the amount of toner is kept at a fixed value. However, since only toner mixing ratio deals with a variation in various conditions, the toner mixing ratio is liable to become abnormal to possibly result in fog and adhesion of carrier.
More specifically, when the humidity of surroundings is lower, toner is easily charged by friction with carrier, and therefore, toner is replenished in a manner to raise a toner mixing ratio by a control device since the amount of toner which adheres to an electrostatic latent image decreases in quantity. On the other hand, when a humidity is higher, toner is not easily charged, and the amount of toner which adheres to an electrostatic latent image increases in quantity. In this case, however, the toner mixing ratio can not be lowered since the toner already replenished can not be removed even if the toner replenishing operation is stopped, and developing operation in proper density can not be performed until the toner already replenished is completely consumed.
(3) Combined Method of ATDC and AIDC
This is a method which is arranged to compensate defective points of the above two methods, wherein the toner mixing ratio in a developer is always kept at a fixed value by the ATDC method, and the amount of toner adhesion (image density) is kept at a fixed value by the AIDC method by varying developing condition.
As a method of varying a developing condition, there is a method to vary rotational numbers of a developing sleeve which is provided for holding and transporting a developer for a developing operation. However, there arise problems in edge effects, reproduction of narrow line, adhesion of carrier, and the like. Moreover, the control system becomes complicated and increases manufacturing costs.
In order to deal with the problems, there is proposed a method of varying developing potential. More specifically, there is a method of controlling an electric potential of latent image to be developed on a photoconductor by varying an amount of electric charge to be charged on the photoconductor or by varying an amount of exposure to the photoconductor, and a method of varying a developing bias potential to be impressed on a developing device which is provided for developing a latent image. By employing the method of varying a developing bias, developing operation can be performed in the most stabilized state since the former method is hard to control in a stabilized state because of dark decay and quantity of light-surface potential characteristics.
In an image density detection process in the AIDC method, an amount of toner adhesion is detected by detecting an irregularly reflected light with a light receiving element upon irradiating infrared rays to a standard pattern since any colored toner can be reflected more than 90% if a light is of long wavelength. As shown in FIG. 6, an output of a sensor tends to be saturated when an amount of toner adhesion exceeds a certain quantity. This is because when more than one layer of toner is stuck, the surface area of the toner is not practically changed thereafter.
When a detection is made in the vicinity of one layer of toner adhesion in the AIDC method, a variation in sensor output to a variation in the amount of adhesion is so small that it is difficult to detect a variation in density. In order to solve this problem, a standard pattern is, therefore, formed in halftone pattern in the AIDC method, and developing bias is controlled so as to make an amount of adhesion at a fixed value assuming an amount of adhesion from the sensor output. However, since the halftone pattern is formed by making an amount of exposure (laser power) smaller, an area which largely varies in quantity of light amount-surface potential characteristic (LDC characteristic) is used as illustrated in FIG. 7, and the surface potential in the pattern portion is largely varied with a little variation in the quantity of light to result in lowering the accuracy in the pattern density, and eventually, the density detection is erroneously conducted to disorder an image.